This achievement that was conducted by the project leader, Dr. Matej Hočevar (the first author of the paper) in collaboration with the experts in the field of laser surface functionalization and nanobiology/nanotoxicology explains how the modification of the stainless steel with nanosecond pulses of Nd:YAG laser influences the interaction between a functionalized metal surface and the osteosarcoma cells. The results show that nanostructured oxide surface reduces cell adhesion and results in cellular stress. We have shown that the initial interaction (within first 24 h) between the cells and the laser-textured surfaces leads to round shaped cells with a smaller footprint. Contrarily, on the non-processed stainless-steel and control-glass surfaces the polygonal, highly elongated, and flattened cells are observed. The cells on the laser-textured surfaces are less dendritic, with short tubular protrusions and an overexpression of extracellular vesicles, which are rarely found on non-treated and control samples. This likely happens due to the formation of nanostructured, high-temperature oxides that are induced by laser ablation. The analysis by X-ray photoelectron spectroscopy reveals that the laser-textured stainless-steel surfaces contain Cr hexavalent oxide, which is more toxic than the native oxide layer on the non-processed samples. The presented results show that osteoblast cells respond to a combination of modified surface topography, roughness, wettability, and chemistry already within 24 h. Thus, they endorse laser surface engineering as an appropriate method for surface modification to control the surface biocompatibility.
COBISS.SI-ID: 14290947
This achievement that was conducted by the project leader, Dr. Matej Hočevar (the leading author of the paper) in collaboration with the experts in the field of laser surface functionalization explains long-term influence of laser-processing parameters on (super)hydrophobicity development and stability of stainless-steel surfaces. Controlling the surface wettability represents an important challenge in the field of surface functionalization for biomedical aplication. Here, the wettability of a stainless-steel surface is modified by 30-ns pulses of a Nd:YAG marking laser (? = 1064 nm) with peak fluences within the range 3.3–25.1 J cm-2. The short- (40 days), intermediate- (100 days) and long-term (1 year) superhydrophilic-to-(super)hydrophobic transition of the laser-textured surfaces exposed to the atmospheric air is examined by evaluating its wettability in the context of the following parameters: (i) pulse fluence; (ii) scan line separation; (iii) focal position and (iv) wetting period due to contact angle measurements. The results show that using solely a short-term evaluation can lead to wrong conclusions and that the faster development of the hydrophobicity immediately after laser texturing usually leads to lower final contact angle and vice versa, the slower this transition is, the more superhydrophobic the surface is expected to become (possibly even with self-cleaning ability). Depending on laser fluence, the laser-textured surfaces can develop stable or unstable hydrophobicity. Stable hydrophobicity is achieved, if the threshold fluence of 12 J cm-2 is exceeded. We show that by nanosecond-laser texturing a lotus-leaf-like surface with a contact angle above 150o and roll-off angle below 5o can be achieved.
COBISS.SI-ID: 16343835
The achievement of the postdoctoral project leader (Dr. Matej Hočevar) in collaboration with experts in the field of laser surface functionalization and heat transfer, makes an important contribution to understanding the influence of critical heat flux density on changes in surface chemistry and morphology of laser structured surfaces. Stability of functionalized surfaces is an often-neglected topic in phase-change heat transfer research. Here, we examine the chemical and morphological changes of textured surfaces on the molecular and atomic level after the critical heat flux incipience during saturated pool-boiling of water. SEM imaging, EDS, AES and XPS analyses are used to examine the surface changes. Copper samples were laser textured via ablation using a nanosecond fiber laser under air or argon atmosphere. Multiscale microcavities, which serve as preferential nucleation sites, were produced on the samples, which exhibited significantly enhanced heat transfer performance in pool-boiling tests. Repeated formation of a vapor film and accompanying temperatures of up to 320?°C during the tests resulted in changes of the surface chemistry and nanomorphology. It was determined that Cu (II) oxide and hydroxide transform into Cu (I) oxide and Cu metal as a result of repeated low-temperature annealing of the surface when a vapor film is formed during the transition towards film boiling. This additionally causes a wettability transition of the functionalized surfaces from hydrophilic towards hydrophobic. Both effects importantly influence the solid-liquid-vapor interface during phase-change heat transfer. Overall, surfaces functionalized via laser texturing exhibited significantly enhanced stability and boiling heat transfer performance.
COBISS.SI-ID: 16653083
The achievement of a postdoctoral project leader, Dr. Matej Hočevar in collaboration with experts in the field of laser surface functionalization and cavitation represents an important contribution to the understanding of the cavitating flow against functionalized surfaces. This contribution clarifies the interaction between liquid flow and solid boundary, that can result in cavitation formation when the local pressure drops below vaporization threshold. The cavitation dynamics does not depend only on basic geometry, but also on surface roughness, chemistry and wettability. From application point of view, controlling cavitation in fluid flows by surface functionalization is of great importance to avoid the unwanted effects of hydrodynamic cavitation (erosion, noise and vibrations). However, it could be also used for intensification of various physical and chemical processes. The results prove that cavitation characteristics significantly depend not only on the surface roughness, but also on the surface wettability. The increased wettability (lower apparent contact angles) delays the incipient cavitation, since the liquid tends to stay in a contact with the hydrophilic surface. Thus, the liquid separates from the surface at higher flowrates (representing higher energies). Here, the development of the surface wettability on the laser- textured metallic surfaces by time offers an interesting approach to test the interaction between the fluid flow and the curved surfaces with the same microtopography, but different wettability. In this work, the surfaces of 10- mm stainless steel cylinders are laser textured in order to demonstrate how hydrodynamic cavitation behavior can be controlled by surface modification. By using nanosecond-laser texturing, surfaces with five different topographies and different wettability were produced and tested in a cavitation tunnel. Cavitation characteristics behind functionalized cylindrical surfaces were monitored simultaneously by high-speed visualization (20,000 fps) and high frequency pressure transducers. The results clearly show that cavitation characteristics differ significantly between different micro-structured surfaces. On some surfaces, incipient cavitation is delayed and cavitation extent decreased in comparison with the reference – a highly polished cylinder.
COBISS.SI-ID: 17154075
Functionalized interfaces enhancing phase-change processes have immense applicability in thermal management. Here, a methodology for fabrication of surfaces enabling extreme boiling heat transfer performance is demonstrated combining direct nanosecond laser texturing and chemical vapor deposition of a hydrophobic fluorinated silane. Multiple strategies of laser texturing are explored on aluminum with subsequent nanoscale hydrophobization. Both superhydrophilic and superhydrophobic surfaces with laser-engineered microcavities exhibit significant enhancement of the pool boiling heat transfer. Surfaces with superhydrophobic microcavities allow for enhancements of the heat transfer coefficient of over 500%. Larger microcavities with a mean diameter of 4.2 um, achieved using equidistant laser scanning separation, induce an early transition into the favorable nucleate boiling regime, while smaller microcavities with a mean diameter of 2.8 um, achieved using variable separation, provide superior performance at high heat fluxes. The enhanced boiling performance confirms that the Wenzel wetting regime is possible during boiling on apparently superhydrophobic surfaces. A notable critical heat flux enhancement is demonstrated on superhydrophobic surfaces with engineered microstructure showing definitively the importance and concomitant effect of both the surface wettability and topography for enhanced boiling. The fast, low-cost and repeatable fabrication process has great potential for advanced thermal management applications.
COBISS.SI-ID: 14158851